The CAS Registry Mumber 135583-61-8 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,3,5,5,8 and 3 respectively; the second part has 2 digits, 6 and 1 respectively. Calculate Digit Verification of CAS Registry Number 135583-61: (8*1)+(7*3)+(6*5)+(5*5)+(4*8)+(3*3)+(2*6)+(1*1)=138 138 % 10 = 8 So 135583-61-8 is a valid CAS Registry Number.